In 1971, Professor Flemings’ student DB When Spencer studied the hot cracking problem of alloy at high temperature, due to the operation error, the original rotational viscometer changed from rotating the outer cylinder of the viscometer when the alloy cooled slowly. Unexpectedly, the shear stress of the latter was reduced by nearly three orders of magnitude compared with the former, and the primary phase of the latter alloy was nearly spherical structure The concept of semi-solid metal forming is put forward for the first time.
Semi solid metals commonly used in semi-solid die casting have the following characteristics:
(1) The temperature of semi-solid metal accords with a certain range, in which the metal is in semi-solid state;
(2) The solid-liquid phase is evenly distributed without obvious solid-liquid segregation;
(3) Different from the traditional liquid forming, the primary A1 particles in semi-solid slurry are nearly spherical.
The semi-solid reward has rheological property. The semi-solid slurry within a certain solid fraction can be moved and clamped as a whole. At the same time, it has certain liquid characteristics, such as easy cutting and flowing. According to the above two characteristics, semi-solid slurry can be effectively transferred and effectively applied force during the forming process to fill the mold cavity.
Generally, the alloy suitable for semi-solid forming needs to meet the requirements of large process window, that is, the sensitivity of fraction change with temperature is low. Therefore, Al Si alloy with wide solid-liquid two-phase region is widely used in semi-solid forming technology. Heating and cooling are two important ways to prepare semi-solid metals, as shown in Figure 1.1. The semi-solid slurry can be obtained by applying specific shear action in the process of cooling, and then heating again by technical means after the molten metal is cut into pieces as required in the process of cooling. The forming method of this slurry is called thixoforming. The semi-solid slurry prepared by a single cooling process of liquid metal melt is called rheological forming. Direct die casting is a method of forming semi-solid slurry prepared by rheological forming, which is called semi-solid Rheo-diecasting. Because of its high formability, automation, high efficiency and low cost, it is widely used in industry.
Thixoforming technology has been developed for a long time. It can be used to prepare automobile parts widely used in industry, such as wheel hub and impeller. Compared with thixoforming, rheological forming is difficult to control because it is difficult to prepare high solid semi-solid slurry, so it is widely used in industry. However, the mechanical properties of die-casting products prepared by rheo forming are between thixoforming products and traditional casting products. Compared with thixo rheo forming, it has huge cost advantages, and compared with traditional casting, rheo forming has the advantages of high performance.